1. Field of the Invention
The invention disclosed relates to trip mechanism for overload relays.
2. Discussion of the Related Art
Overload relays are intended to protect motors conductors against excessive heating due to prolonged motor overcurrents up to and including locked rotor currents. Overload relays are distinguished from circuit breakers, in that circuit breakers typically protect other types of branch-circuit components from higher currents acting over a shorter interval, due to short circuits or grounds.
Thermal overload relays sense prolonged motor overcurrent by converting this current to heat in a resistance element. The heat generated is used to open a normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Generally, there are three types of overload relays, the melting alloy thermal overload relay, the bimetallic thermal overload relay, and the solid state overload relay.
In melting alloy thermal overload relays, the motor current passes through a small heater winding. Under overload conditions, the heat causes a special solder to melt, tripping the relay and opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Bimetallic thermal overload relays employ a bimetal strip associated with a current carrying heater coil. When an overload occurs, the heat will cause the bi-metal to deflect and trip the relay, opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
Solid state electronic overload relays do not require thermal units, instead use current transformers that respond directly to the motor current. Once an overload condition is reached, the electronic circuit of the overload relay trips, causing the contacts to open in a manner similar to the bimetallic thermal overload relay, opening the normally closed contact in series with a starter coil causing the motor to be disconnected from the line.
The normally closed contact in existing overload relays is typically driven by a mechanical bi-stable spring that is tripped by a complex sequence of levers that are difficult to manufacture because of the tolerances they require. Spring actuated bi-stable mechanisms can be difficult to dimension correctly making it difficult to guaranty consistent tripping positions and contact forces. What is needed is a simplified overload tripping mechanism the replaces the mechanical bi-stable spring with a mechanism that does not require difficult manufacturing steps.